The invention relates to dust/particle monitors, and particularly but not necessarily exclusively to a method of monitoring fine particles harmful to humans in working environments.
As a result of industrial processing of materials, fine particulate is generated which may be harmful to a workforce. Particulate less than 7.5 .mu.m in diameter attaches to the human pharynx, bronchi and alveoli, leading, after long term exposure, to several serious respiratory conditions. Government places strict Occupational Exposure Limits (based on a time weighted average) on employers to restrict the amount of airborne contamination a worker may inhale over a given time period.
The recognised technology used for measuring the concentration of fine particulate inhaled consists of a regulated vacuum pump (flow rate 2-4 liters per minute) worn on a harness connected by means of a tube to a sampling head placed within a persons breathing zone to sample the micro-environment to which the person is exposed.
The air is drawn into the sampling head and through a pre-weighed filter paper by the pump. As particulate is deposited on to the filter paper, the flow rate diminishes due to increasing back pressure and the speed of the pump is regulated to keep the flow rate constant. At the end of a period of time, the filter is removed, dried and re-weighed on a micro-balance. The weight of the filtered particulate is calculated and then using the following calculation, the time weighted average concentration is derived. ##EQU1##
Where
W=weight of particulate PA1 F=flow rate in 1/min PA1 ST=sampling time PA1 1. No instantaneous warning of excessive exposure either due to very high immediate levels or long term excessive exposure. PA1 2. Bulky sampling equipment with associated high noise levels. PA1 3. Errors introduced into results due to poor handling, drying and weighing of filter media. PA1 4. Fluctuating flow rate due to use of diaphragm (reciprocating) vacuum pumps and inadequate pump speed controllers lead to inaccurate flow rate estimation and inefficient cyclone separation efficiency.
To limit the particulate size range into the sampling head to a desired range (normally Total Respirable Dust up to 7.5 .mu.m) the air containing the dust is first passed through a cyclone separator so as to remove dust particles above 7.5 .mu.m. The volumetric air flow rate through a cyclone separator must run at a specific volumetric flow rate in order to separate particles above a given size accurately.
The disadvantages of the present method are:
The mechanism of detecting air borne particulates, using charge induction techniques, involves causing the air or gas in which the particulates are suspended to pass through a duct or pipe in which there is an associated sensing electrode or electrodes. Provided there is some charge on a particle the movement of the particle pass the electrode causes a varying charge to be induced on the electrode which causes a varying current to flow in the electrode which may be detected using appropriate electronic signal processing.
The mechanisms whereby particles become electrically charged are many, and the magnitude of charge associated with a particle depends on many factors such as particle size, particle shape, particle material, the level of moisture, and particularly the charging history of the particle. In many industrial processes, where dusts are formed during the processing and/or movement of bulk solid materials, the magnitude of charges produced on the particles is of a sufficient level for detection.
On the other hand where dust has rested on a surface for some time the charge may be partially or completely lost due to conduction of the charge to ground. If the dust is then lifted into the air by air movement over the dust laden surface the magnitude of charge on the particles may be too small to measure.
Because of the variable nature of the magnitude of the charges resident on air borne particles, methods of estimating the quantity of dust based on overall charge values are subject to considerable and unknown errors. This problem is further aggravated in that the mechanism of sensing charge induced onto electrodes involves a process of differentiation (as described below) so that only a change in induced charge in the sensing electrode can be measured.
In the case where a cloud of particles is passing the electrode, the induced charge on the sensing electrode is the sum of all of the individual induced charges from each particle and the output voltage from the signal processing system will be proportional to the derivative of the summed charge signal.
The resulting output voltage will be a noise signal which can be processed on-line by deriving, for example, the rms value, or the absolute value of the signal. However, these parameters of the noise signal are only loosely related to the total charge on the dust cloud due to the inherent differentiation process relating the current flowing in the electrode to the induced charge.
Not-with-standing these inherent problems, in the past attempts have been made to measure dust concentrations using these techniques by calibrating instruments with known concentration levels of dust and particulates.
The difficulty with measurements based on this approach (apart from the differentiation problem mentioned above) is that the calibration is not stable because the factors which determine the magnitude of charge on the particles may vary with time.
An illustration of the method of detecting particles using electrostatic techniques is shown in FIG. 1 which is a schematic sectional elevation of the volume below a charge distribution curve. A single particle 1 at position A has an electric charge of +q coulombs on it, and it is at a distance of d from the duct wall 2 and a lateral distance of 1 from the position of the sensing electrode 3. The duct wall is an electrical conductor and is at earth potential. The sensing electrode is attached to the outside surface of an insulating section 4 of the duct wall and is connected to the inverting input of an operational amplifier 5 which will also be at earth potential as it is a virtual earth point. The electrode is shown as mounted on the outside surface of the insulating section so as to avoid the possibility of particles colliding with the electrode and causing charges to be placed directly on the electrode by the tribo-electric effect. Alternatively the electrode may be mounted on the inside surface but with a thin layer of insulation.
The charge of +q Coulombs on the particle will cause there to be a charge of opposite polarity induced on the earthed conducting surfaces of the duct wall and the sensing electrode (which is also at earth potential) as shown by the curve 6 of FIG. 1. The distribution of the charge on the duct wall 2 and electrode 3 is as shown by the curve 6 of FIG. 1, and the total charge induced on the duct wall and the sensing electrode is represented by the area under the curve 6 and will be -q Coulombs. The fraction of the charge that appears on the sensing electrode 3 will be equal to the ratio of the shaded volume of the charge distribution immediately above the electrode to the total volume under the charge distribution curve.
If we now consider the particle to be moving at constant velocity to the right and parallel to the plate, the fraction of the charge intercepted by the sensing electrode 3 will vary as a function of time as shown in the graph of FIG. 2(a).
As a result of the charge varying on the electrode with time and as is indicated in FIG. 1, a current I.sub.e will flow between the electrode and the virtual earth of the amplifier according to the equation: EQU I.sub.e =dq/dt (1)
The current waveform is shown in FIG. 2(b) and will produce a voltage Vo at the output of the operation amplifier which is given by the relationship: EQU Vo=-R.I.sub.e (2)
The voltage waveform Vo shown in FIG. 2(c) will be the same shape as the current waveform I.sub.e shown in FIG. 2(b), but it will be inverted due to the action of the operational amplifier.
In effect, the operational amplifier output voltage waveform Vo is equivalent to the time derivative of the charge waveform intercepted by the sensing electrode. The output voltage of the operational amplifier will have fallen to approximately a tenth of the maximum signal zero when the particle is at a position where the lateral displacement is three times the distance between the particle and the duct wall as discussed above.
The difficulty in this approach is that the calibration is not stable. The reasons for this are that the electric charge on each particle depends on several factors mentioned above that can not be controlled or independently measured.